A community is a group of several populations living and interacting with each other in an ecosystem. A community is many species living together, whereas the term population refers to just one species

• A habitat is the location in which a community, species, population or organism lives.

• A habitat of a species includes geographical and physical locations and the type of ecosystem required to meet all environmental conditions needed for survival

• An interdependent organisms (the biotic component) and the physical environment (the abiotic component) with which it interacts. 

• Ecosystems are open systems

  • Both energy and matter can enter and exit the system. These can be lost (law of thermodynamics) or passed to another ecosystem

• There are three major types of ecosystem:

Sustainability is a natural property of ecosystems

• Ecosystem are in their steady-state equilibrium.

• Energy released (leaving an ecosystem) from cellular respiration is balanced by the input of the solar energy

In general, a sustainably ecosystem shows these features:

1. Balanced energy and carbon recycling

   • Input from solar energy. Energy is used by plants and other photosynthetic organisms to convert carbon dioxide molecules to glucose (sugar) molecules. 

   • Sugar is used in cellular respiration (with or without oxygen). Energy is obtained by breaking down the sugar molecules. As energy is used and heat is lost, carbon dioxide is released back to the atmosphere. The sugar molecules can also be used to increase biomass (growth in terms of the size of the organism). 

   • As an organism dies, their carbon based matters are decomposed by decomposers (such as bacteria and fungi) and carbon is returned to the atmosphere. Note that decomposition includes cellular respiration.

2. Water cycle

   • Water falls of as rain from the cloud. This is called precipitation. Water molecules can also precipitate in a form of snow although is is caused by crystallisation of water vapour in the atmosphere. 

   • As water reached the soil and water bodies (rivers, lakes, oceans etc), the water is now available for uptake by organisms. Plant would take up water for photosynthesis and respiration (and other metabolic processes) while animals drink water for cellular respiration (and other metabolic processes). The water molecules will be returned to the environment in a form of waste materials (such as pee and sweat) and as water vapour (by products of respiration)

3. Nutrients recycling

   • Nutrients such as nitrates are recycled in an ecosystem. The recycling involves several biotic and abiotic factors. 

   • Nitrogen cycle for example involves the conversion of atmospheric nitrogen to nitrates in the soil. This requires large energy such as the lighning and nitrogen fixing bacteria in the soil and roots of plants. Once nitrates are formed, they are now available for intake by plants for plant growth. The nitrates will later be consumed by the consumers along a food chain. As consumers die, their nitrates will be returned to the soil during decomposition process.

Throughout the above sustainable processes, matters and energy are also passed to another ecosystem. For example:

• Rain may carry some nutrients by runoff to another habitat

• Nutrient leaching transfers nutrients underground from one ecosystem to another

• Energy and matter transferred from marine to terrestrial. For example, birds consumer fish